This is the coolest. Official Xlink Kai Forums has a tutorial online complete with screenshots showing you how to set up your Mac to act as an Adhoc network for your PSP, so that you can play PSP games capable of multiplayer over the internet using Xlink Kai’s free online gaming service. All you need is a free account with Xlink Kai, a copy of Amaryllis from Nullriver (the makers of PSPware), and a copy of Kaid OS X Menu from Xlink.
In order to make everything work, you have to have your Mac connected to the internet via your Ethernet connection, then create a computer to computer network between you Mac and your PSP, then launch Kaid and launch Amaryllis, connect to the game room where you want to play, and then launch the multiplayer option of the game you want to play on your PSP and either host or search for games.
I gave this a shot last night and raced several times in Wipeout Pure with an online opponent. It was good, but there was some lag, so my opponent would intermittently disappear or only appear as a non-moving ghost. I also attempted to play Untold Legends, but every game I connected to dropped shortly after connecting. I have no idea of knowing if this was just a temporary problem, or if the service is still in its Alpha stages, but it’s encouraging at least to know that people are figuring out ways to tunnel these games over the internet and that they’re including the Mac in this process.